HC series is not level shifting, they have full protection diodes on theinputs - trying to feed them 5V signals whilst powered from 3.3V will pull the 3.3Vrail up to 4.5V or so and probably damage something...
Input protection circuits are different from those ofthe high speed CMOS IC's. The Vcc side diodes are designed to allowlogic-level conversion from high-level voltages (upto 13V) to low level voltages.
Can the 3.3v output be controlled by the microprocessor or is it always on? I need it to control a relay. Thanks!
Control Voltage Range 3 - 32 DCControl Current Max. 34 mAInput Resistance 900 Ohm
Project is based on an arduino mega, which has to run at 16Mhz, therefore 5V power supply is mandatory. At the same time, I want to use one of the 2.4 inch ILI9341 SPI displays, which come with a built in SD card reader, which is nice.These displays have a 5V to 3.3V power converter on board. However they are very cheaply made, and the signal level shifting is done with in series resistors. (implicitly relying on the built in clamping diodes of the display and the SD to do the level shifting)This didn't work on some SD cards for me. I had sporadic errors, and downright failing SD cards.With a level shifter, everything is fine now. The in series resistor on the display PCB doesn't harm.The LCD modules and SD modules from adafruit, sparkfun etc. all have level shifters on their PCB (either with the 4050 IC, or discrete with transistors), so I believe this is a tried and tested method to connect 3.3V peripheral equipment to 5V microcontrollers.
That certainly is the most reliable method, particularly if you are going to use the maximum speed with hardware SPI. For less than a Megahertz, the diodes and resistors (not resistive dividers) should work. And these "super diode" level shifters:http://www.ebay.com/itm/5PCS-5V-3V-2-CH-I2C-IIC-Logic-Level-Converter-Module-Bi-Directional-for-Arduino-/401234225236
The arduino board has a 3.3V voltage regulator on board. The pin is the output of this voltage regulator. Therefore this pin can not be controlled by the microprocessor.However I doubt you need 3.3V to control a mechanical relay. What relay are you talking about? Please post a link to the technical specification of your relay.I am speculating here that you actually want to control a SSR relay. Here's a random SSR datasheet:http://cdn-reichelt.de/documents/datenblatt/C300/WGA%205-6D25.pdfThe relevant info:This means ANY voltage between 3 and 32 V will be okay to control the SSR.With 5V (output voltage of an Output pin), the SSR will draw I = U/R = 5V/900 Ohm = 5.6 mAThe Arduino can supply max 200mA alltogether, and max. 20mA per Pin (if I remember the datasheet correctly). So as long as you don't have other big loads on the arduino, you can connect this kind of SSR directly.You need a microprocessor output pin to control your relay. More information:https://www.arduino.cc/en/Tutorial/DigitalPinsThomas
I plan on connecting a temperature sensor to the arduino (using the 5v out)
I need the arduino to send a signal to the relay (which is where I would think it would connect to the 3.3v)